1. Field of the Invention
The present invention relates to a wear resistant sintered member which is superior in wear resistance at high temperatures, and in particular, relates to a technique suited to be used for a valve seat insert of internal combustion engines.
2. Description of the Related Art
In order to deal with performance enhancement and power increase of engines for automobiles, a sintered alloy for a valve seat insert having high wear resistance and high strength at high temperature has been required, and the present applicants have also developed a wear resistant sintered alloy (Japanese Patent Publication No. 55-36242) manufactured by a method disclosed in Japanese Patent No. 1043124. In addition, the applicants further developed wear resistant sintered alloys which are superior in high wear resistance and high strength at high temperature, as disclosed in Japanese Patent Publication No. 5-55593, Japanese Patent Application Laid-open No. 7-233454, and the like, in order to deal with recent even greater performance enhancement, power increase, and in particular, increase in combustion temperature due to lean combustion. However, the above conventional materials were disadvantageous in cost because expensive Co-based materials were employed as a hard phase in order to improve the performance at high temperature.
It is an object of the present invention to provide a wear resistant sintered member which can exhibit superior wear resistance at the same level as those of the conventional materials without using a hard phase consisting of Co-based materials.
In order to solve the above problems, a first embodiment of a wear resistant sintered member according to the present invention exhibits a metallographic structure comprising a first hard phase and a second hard phase diffused in an Fe-based alloy matrix, wherein the first hard phase comprises Mo silicide particles dispersed in an Fe-based alloy matrix of the first hard phase, the second hard phase comprises a ferrite phase or a mixed phase of ferrite and austenite having a higher Cr concentration than the Fe-based alloy matrix surrounding a core consisting of Cr carbide particles, the Mo silicide particles in the first hard phase are contained in an amount of 3 to 25% by area in the member, and the Cr carbide particles in the second hard phase are contained in an amount of 3 to 30% by area in the member. FIG. 1 shows a schematic drawing of the metallographic structure.
{circle around (1)} First Hard Phase
As shown in FIG. 1, in the first hard phase, Mo silicide is dispersed in an Fe-based alloy matrix of the first hard phase, and moreover, composite silicide composed of Mo, Fe, Cr, or Ni, or intermetallic compounds of these elements, may be partially dispersed instead of the Mo silicide. Mo silicide is hard so as to have an effect which improves wear resistance of the wear resistant sintered member, and it has solid lubricity so that action (facing member interaction) which wears or attacks a facing material is low.
In addition, it is preferable that the alloy matrix of the first hard phase for dispersing Mo silicide, etc., be composed of an alloy consisting of Fe and at least one of Ni and Cr. Wear resistance of the first hard phase can be further improved by strengthening the alloy matrix of the first hard phase. Furthermore, Ni or Cr in the alloy matrix of the first hard phase has an effect in which adhesion to the alloy matrix is further strengthened by diffusing into the surrounding matrix.
The Mo silicide particles must be dispersed in the matrix of the first hard phase of the wear resistant sintered member in an amount of 3 to 25% by area. Here, the xe2x80x9careaxe2x80x9d of the Mo silicide particles refers as an inside area of an outline of the Mo silicide particles. When it is under 3% by area, an improvement effect is poor, and in contrast, when it exceeds 25% by area, facing member interaction increases, and the facing member is thereby worn.
{circle around (2)} Second Hard Phase
As shown in FIG. 1, the second hard phase is a phase in which a ferrite phase or a mixed phase of ferrite and austenite, having a higher Cr concentration than the matrix, surrounds a core consisting of Cr carbide particles. Since Cr carbide as a core receives impacts in a valve seating and the surrounding mixed phase of austenite and ferrite has a buffering effect, wear resistance is improved. In addition, Cr which further diffuses contributes to improvement of wear resistance of the overall sintered alloy by acting to strengthen the matrix or the second hard phase as described below. Furthermore, when carbide particles of Mo, V, or W, are dispersed in addition to Cr carbide particles in the second hard phase, it is effective to further improve wear resistance.
The Cr carbide particles must be dispersed in the matrix of the second hard phase in an amount of 3 to 30% by area. Here, an area of the Cr carbide particles refers as an inside area of an outline of the Cr carbide particles. When it is under 3% by area, the above effect is poor and does not contribute to wear resistance, and in contrast, when it exceeds 30% by area, wear of a facing material is enhanced by hard Cr carbide, etc., and worn powder of a facing material acts as grinding particles, so that the sintered member also is worn.
Component composition and metallographic structure of the matrix in a wear resistant sintered member of the present invention are not limited, and conventional alloys can be employed.
In order to solve the above problem, a second embodiment of a wear resistant sintered member according to the present invention has an overall composition comprising, by mass, Mo: 1.25 to 17.93%, Si: 0.025 to 3.0%, C: 0.35 to 0.95%, at least one of Cr: 0.025 to 3.0% and Ni: 0.025 to 3.0%, and a balance of Fe and unavoidable impurities, and exhibits a metallographic structure comprising a matrix which consists of bainite or a mixture of bainite and martensite, and a first hard phase comprising Mo silicide particles dispersed in an alloy matrix which consists of Fe and at least one of Ni and Cr, wherein the Mo silicide particles are contained in the alloy matrix of the first hard phase in an amount of 3 to 30% by area.
FIG. 2 shows a schematic drawing of a metallographic structure of the second embodiment of a wear resistant sintered member according to the present invention. As shown in FIG. 2, in the second embodiment of a wear resistant sintered member of the present invention, the above first hard phase is strengthened by Ni and/or Cr, the composition of the matrix comprises, by mass, Mo: 0.8 to 4.2%, C: 0.35 to 0.95%, and a balance of Fe and unavoidable impurities, and the matrix consists of bainite or a mixture of bainite and martensite, and therefore, strength and wear resistance of the matrix are improved and superior wear resistance is exhibited by only the first hard phase.
In the first hard phase, Mo silicide is dispersed in an alloy matrix consisting of Fe and at least one of Ni and Cr. When the Mo silicide particles are dispersed in the alloy matrix of the first hard phase in an amount of less than 3% by area, the improvement effect of the wear resistance is insufficient. In contrast, the upper limit of the content of the Mo silicide particles in the first hard phase is higher than that of the above embodiment of a wear resistant sintered member since the second embodiment has no second hard phase; however, when it exceeds 30% by area, the facing member interaction increases and a facing member is thereby worn.
The matrix has a single phase structure consisting of bainite which has high strength, which is hardest after martensite, and which is superior in wear resistance, or has a mixed structure of the above bainite and martensite which is the hardest structure and which has a high facing member interaction. In the mixed structure, by mixing martensite and bainite, the facing member interaction of martensite is eased and the hardness is moderately reduced, and therefore, the wear resistance is improved. In the matrix in the present invention, since Mo is contained, fine Mo carbide particles precipitate and, the wear resistance is further improved.
A third embodiment of a wear resistant sintered member according to the present invention has an overall composition comprising, by mass, Mo: 1.01 to 15.43%, Si: 0.025 to 2.5%, C: 0.36 to 1.67%, Cr: 0.2 to 7.5%, and a balance of Fe and unavoidable impurities, and exhibiting a metallographic structure comprising an alloy matrix which consists of bainite or a mixture of bainite and martensite, a first hard phase and a second hard phase diffused in the above Fe-based alloy matrix, wherein the first hard phase comprises Mo silicide particles dispersed in an Fe-based alloy matrix of the first hard phase, the second hard phase comprises a ferrite phase or a mixed phase of ferrite and austenite, having a higher Cr concentration than the alloy matrix, surrounding a core consisting of Cr carbide particles, the Mo silicide particles are contained in the first hard phase in an amount of 3 to 25% by area, and the Cr carbide particles are contained in the second hard phase in an amount of 3 to 30% by area.
FIG. 3 shows a schematic drawing of a metallographic structure of the third embodiment of a wear resistant sintered member according to the present invention. As shown in FIG. 3, in the third embodiment of a wear resistant sintered member of the present invention, a second hard phase in a wear resistant sintered member of the above first embodiment is diffused in a wear resistant sintered member of the above second embodiment, and the upper limit of the content of the first hard phase is limited in an amount of 25% by area, in order to diffuse the second hard phase.
In a wear resistant sintered member in the third embodiment, it is preferable that at least one of Ni: 0.025 to 2.5% by mass and Cr: 0.025 to 2.5% by mass be added as an overall composition to the above first hard phase, and that the alloy matrix consist of Fe and at least one of Ni and Cr. The wear resistance of the first hard phase can be further improved by strengthening the alloy matrix in the first hard phase. Furthermore, Ni or Cr in the alloy matrix to of the first hard phase has an effect in which adhesion to the alloy matrix is further strengthened by diffusing into the surrounding matrix.
The second hard phase is a phase in which a ferrite phase or a mixed phase of ferrite and austenite, having a higher Cr concentration than the matrix, surrounds a core consisting of Cr carbide particles. The Cr carbide in the second hard phase is hard and contributes to improvement of wear resistance. The ferrite phase or the mixed phase of ferrite and austenite having a higher Cr concentration than the surrounding soft matrix adheres Cr carbide firmly and for example, when the sintered member is used as a valve seat insert, it acts as a buffer material in the seating of a valve which is a facing material, and has an effect which absorbs impacts on the facing material.
When the content of the Cr carbide particles in the second hard phase is under 5% by area, the effect of improvement of wear resistance is very poor, and in contrast, when it exceeds 30% by area, the facing member interaction increases and the facing material is thereby worn. Furthermore, in the case in which the Mo silicide particles in the first hard phase coexist with the second hard phase, when it is contained exceeding 25% by area, facing member interaction of the overall member increases and therefore, the upper limit thereof is set to be 25% by area. In the wear resistant sintered member of the third embodiment, the content of the Mo silicide particles is set to be 5% by area or more in order to exhibit the effect of the first hard phase.
It is preferable that hardness of the Mo silicide particles of the first hard phase in the above wear resistant sintered members of the first to third embodiments described above be MHV ranging from 600 to 1400. When the hardness of the Mo silicide is low, the effect of improvement of the wear resistance is insufficient, and in contrast, when it is excessively high, the facing member interaction increases and the wear of the facing member is promoted. Therefore, it is preferable that the hardness of the first hard phase consisting of the Mo silicide be MHV of 600 to 1400.
Mo: Mo contributes to the formation of the first hard phase which is superior in wear resistance by forming Mo silicide as described above. Furthermore, the matrix is solid-solution-strengthened by dissolving Mo therein in addition to the formation of the above silicide and the matrix structure thereby consists of a bainite phase or a mixed phase of bainite and martensite and Mo also contributes to improving the wear resistance of the matrix. When the content of Mo is low, the strengthening effect of the matrix or precipitation amount of Mo silicide is reduced, and an improvement effect on wear resistance is decreased. In contrast, when Mo is contained in excess, the precipitation amount of Mo silicide is too much or the matrix becomes too hard, facing member interaction increases, and wear of a facing material thereby increases. Therefore, in the case of the second embodiment of a wear resistant sintered member of the present invention, the Mo content of 1.25 to 17.93% by mass is preferred, and in the case of the third embodiment thereof, the Mo content of 1.0 to 15.43% by mass is preferred.
Si: Si contributes to improving wear resistance by reacting with Mo to form hard Mo silicide of the first hard phase. When the content of Si is low, silicide is not sufficiently precipitated. In contrast, when Si is contained in excess, the compressibility is reduced due to powder hardening, and the adhesion to the matrix is reduced by firmly forming an oxide film on the surface of the powder. Therefore, in the case of the second embodiment of a wear resistant sintered member of the present invention, the Si content of 0.025 to 3.0% by mass is preferred, and in the case of the third embodiment thereof, the Si content of 0.025 to 2.5% by mass is preferred.
Cr: Cr is selectively added to the first hard phase with Ni as described below, and in the third embodiment of a wear resistant sintered member, it is also added to the second hard phase.
Cr in the first hard phase has an effect in which the hardness of the first hard phase is increased by strengthening the alloy matrix of the first hard phase, and thereby the wear resistance is improved and the falling off of the Mo silicide is prevented. In addition, it also has an effect in which the adhesion to the matrix is improved by dispersing in the matrix structure. Therefore, by these effects, it contributes to the improvement of the wear resistance. When the content of Cr contained as a first hard phase is low, the above effects which act in the hard phase are insufficient. In contrast, when Cr is contained in excess therein, the compressibility is reduced due to powder hardening, and the adhesion to the matrix is reduced by firmly forming an oxide film on the surface of the powder. Therefore, in the case of the second embodiment of a wear resistant sintered member of the present invention, it is preferable that the content of Cr contained as a first hard phase be 0.025 to 3.0% by mass in overall composition, and in the case of the third embodiment thereof, it is preferable that it be 0.025 to 2.5% by mass in overall composition.
Cr in the second hard phase forms a second hard phase in which a hard phase consisting of Cr carbide is a core, and thereby the wear resistance is further improved. In addition, Cr which diffused from the second hard phase to the matrix strengthens the adhesion between the hard phase and the matrix, and further strengthens the matrix structure or matrix of the first hard phase, and the hardenability is thereby further improved. Furthermore, it is effective that an area having a high Cr concentration surrounding the second hard phase form ferrite and has an effect which buffers an impact in a valve seating and which prevents hard components such as Cr carbide, etc., from falling off on a wear sliding surface. When the content of Cr contained as a second hard phase is low, the above effects which act in the hard phase are insufficient. In contrast, when Cr is excessively contained therein, the compressibility is reduced due to powder hardening, and the adhesion to the matrix is reduced by firmly forming an oxide film on the surface of the powder. Therefore, it is preferable that the content of Cr contained as a second hard phase be 0.2 to 7.5% by mass in overall composition.
Therefore, in the case in which it is selected as a first hard phase forming element in the second embodiment of a wear resistant sintered member of the present invention, it is preferable that the content of Cr be 0.025 to 3.0% by mass, and in the third embodiment thereof, in the case in which it is not selected as a first hard phase forming element, it is preferable that it be 0.2 to 7.5% by mass, or in the case in which it is selected as a first hard phase forming element, it is preferable that it be 0.225 to 10% by mass.
Ni: Ni is selectively added to the first hard phase with Cr as described above, and has an effect in which the hardness of the first hard phase is increased by strengthening the alloy matrix of the first hard phase, and thereby the wear resistance is improved and the falling off of the Mo silicide is prevented. In addition, it also has an effect in which the adhesion to the matrix is improved by dispersing in the matrix structure. Therefore, by these effects, it contributes to the improvement of the wear resistance. When the content of Ni is low, the above effect is insufficient. In contrast, when Ni is excessively contained therein, the compressibility is reduced due to powder hardening, and the wear resistance is deteriorated by austenitizing the matrix. Therefore, in the case in which it is selected as a first hard phase forming element, in the second embodiment of a wear resistant sintered member of the present invention, it is preferable that the content of Ni be 0.025 to 3.0% by mass, and in the third embodiment thereof, it is preferable that it be 0.025 to 2.5% by mass.
C: C acts to strengthen the matrix and contributes to improvement of the wear resistance. In addition, the third embodiment of a wear resistant sintered member of the present invention also has an effect of contributing to the improvement of the wear resistance by forming Cr carbide. When the content of C contained in the matrix is under 0.35% by mass, ferrite, in which both the wear resistance and strength are low, remains, and in contrast, when it exceeds 0.95% by mass, the strength is reduced due to precipitation of cementite at grain boundaries. Therefore, the content of C contained in the matrix is set to be 0.35 to 0.95% by mass. Furthermore, when the content of C in the second hard phase is under 0.01% by mass, in the overall composition, the carbide is not sufficiently formed and the improvement of the wear resistance is thereby insufficient. In contrast, when the content of C exceeds 0.72% by mass in the overall composition, the wear of a facing member is enhanced by increasing the amount of carbide formed. In addition, the compressibility is reduced by hardening of powder, the strength of the matrix is lowered, and the wear resistance is thereby decreased. Therefore, in the second embodiment of a wear resistant sintered member of the present invention, it is preferable that the content of C be 0.35 to 0.95% by mass, and in the third embodiment thereof, it is preferable that it be 0.36 to 1.67% by mass.
In the above third embodiment of a wear resistant sintered member of the present invention, the wear resistance of the second hard phase can be further improved by containing at least one of, by mass in the overall composition, Mo: 0.09 to 0.15%, V: 0.01 to 0.66%, and W: 0.05 to 1.5% in the second hard phase.
Mo contributes to the improvement of the wear resistance by forming carbide with C in the second hard phase forming powder and by forming a core in the second hard phase which consists of the Mo carbide and the above Cr carbide. In addition, Mo, which did not form the carbide, has an effect in which high temperature hardness and high temperature strength of the second hard phase are improved by dissolving in the second hard phase. When the content of Mo in the second hard phase is under 0.09% by mass in the overall composition, the above effect is insufficient, and in contrast, when it exceeds 0.15% by mass, the wear of a facing member is enhanced by increase in a precipitation amount of the carbide.
V contributes to the improvement in the wear resistance by forming fine carbide with C in the second hard phase forming powder. Furthermore, the above carbide has an effect which prevents Cr carbide from coarsening, the wear of a facing member is suppressed and the wear resistance is thereby improved. When the content of V in the second hard phase is under 0.01% by mass in the overall composition, the above effect is insufficient, and in contrast, when it exceeds 0.66% by mass, the wear of a facing member is enhanced by the increase in the precipitation amount of carbide.
W contributes to the improvement in the wear resistance by forming fine carbide with C in the second hard phase forming powder. In addition, the above carbide has an effect which prevents the Cr carbide from coarsening, and the wear of a facing member is suppressed and the wear resistance is thereby improved. When the content of W in the second hard phase is under 0.05% by mass in the overall composition, the above effect is insufficient, and in contrast, when it exceeds 1.5% by mass, the wear of a facing member is enhanced by increasing of a precipitation amount of the carbide.
The above wear resistant sintered members of the present invention are inexpensive because a Co-based hard phase is not used, and it has a wear resistance at the same level or greater than that of conventional materials.
A first manufacturing process for a wear resistant sintered member of the present invention comprises: mixing a first hard phase forming powder in an amount by mass of 5 to 25% comprising Si: 0.5 to 10%, Mo: 10 to 50%, at least one of Ni: 0.5 to 10% and Cr: 0.5 to 10% as necessary, and a balance of Fe and unavoidable impurities, a second hard phase forming powder in an amount of 5 to 30% comprising Cr: 4 to 25%, C: 0.25 to 2.4%, at least one of Mo: 0.3 to 3.0%, V: 0.2 to 2.2% and W: 1.0 to 5.0% as necessary, and a balance of Fe and unavoidable impurities, and a graphite powder in an amount of 0.35 to 0.95%, with an Fe-based matrix forming alloy powder; compacting in a desired shape; and sintering.
In the above first manufacturing process for a wear resistant sintered member of the present invention, an Fe-based alloy powder is not particularly limited, and conventional powders (an Fe-based alloy powder, a mixed powder of at least two Fe-based alloy powders, a mixed powder or a partially diffused alloy powder between an Fe-based alloy powder or an Fe powder and another metal powder or another alloy powder, etc.), can be employed. In addition, it is suitable that sintering conditions be 1100 to 1200 C. for 30 minutes to 2 hours, which is generally used.
A second manufacturing process for a wear resistant sintered member of the present invention comprises: mixing a first hard phase forming powder in an amount by mass of 5 to 30% comprising Si: 0.5 to 10%, Mo: 10 to 50%, at least one of Ni: 0.5 to 10% and Cr: 0.5 to 10%, and a balance of Fe and unavoidable impurities, and a graphite powder in an amount of 0.35 to 0.95%, with a matrix forming alloy powder comprising Mo: 0.8 to 4.2%, and a balance of Fe and unavoidable impurities; compacting in a desired shape; and sintering.
A third manufacturing process for a wear resistant sintered member of the present invention comprises: mixing a first hard phase forming powder in an amount by mass of 5 to 25% comprising Si: 0.5 to 10%, Mo: 10 to 50%, at least one of Ni: 0.5 to 10% and Cr: 0.5 to 10% as necessary, and a balance of Fe and unavoidable impurities, a second hard phase forming powder in an amount of 5 to 30% comprising Cr: 4 to 25%, C: 0.25 to 2.4%, at least one of Mo: 0.3 to 3.0%, V: 0.2 to 2.2% and W: 1.0 to 5.0% as necessary, and a balance of Fe and unavoidable impurities, and a graphite powder in an amount of 0.35 to 0.95%, with a matrix forming alloy powder comprising Mo: 0.8 to 4.2%, and a balance of Fe and unavoidable impurities; compacting in a desired shape; and sintering.
A fourth manufacturing process for a wear resistant sintered member of the present invention is characterized in that a matrix forming mixed powder which mixes, by mass, an Fexe2x80x94Cr-based alloy powder in an amount 60% or less comprising Cr: 2 to 4%, Mo: 0.2 to 0.4%, V: 0.2 to 0.4%, and a balance of Fe and unavoidable impurities, with an Fexe2x80x94Mo-based alloy powder comprising Mo: 0.8 to 4.2%, and a balance of Fe and unavoidable impurities, is used, instead of the matrix forming alloy powders used in the above first to third manufacturing processes.
In the following, the bases of the numerical limitations of the above component compositions will be explained.
A matrix structure using a matrix forming alloy powder (Fexe2x80x94Mo-based alloy powder) is bainite. Bainite is a metallographic structure having a high hardness and a high strength and is superior in wear resistance. Furthermore, in the present invention, since Mo is contained in the matrix, the wear resistance is also improved by precipitating fine Mo carbide. The above matrix forming alloy powder is also superior in the adhesion in the first hard phase, and it constitutes a matrix of an alloy in the present invention. In addition, when the second hard phase is contained, the hardenability of the matrix is improved by Cr which migrated from the second hard phase, and a mixed phase of bainite and martensite is formed by martensite produced in the region, so that the wear resistance is further improved.
Mo: Mo has an effect in which the matrix is strengthened by dissolving therein and in which hardenability of the matrix structure is improved, and contributes to improving the strength and the wear resistance of the matrix by such effects. Furthermore, the first hard phase forming powder is an Fexe2x80x94Mo-based alloy powder as described below and the matrix forming powder is also an Fexe2x80x94Mo-based alloy powder, and therefore, the adhesion of the first hard phase forming powder to the matrix is superior. However, when the content of Mo is under 0.8% by mass, the strength of the matrix is insufficient, and in contrast, when it exceeds 4.2% by mass, the compressibility is decreased by hardening of the powder. Therefore, the content of Mo is set to be 0.8 to 4.2% by mass.
The matrix forming mixed powder is a mixed powder which mixes an Fexe2x80x94Cr-based alloy powder in an amount of 60% by mass or less with an Fexe2x80x94Mo-based alloy powder used as the above matrix forming alloy powder. In an area using the Fexe2x80x94Cr-based alloy powder, an oxide film is easily formed, and therefore, the clumping resistance is improved, and it is effective for improvement of the wear resistance in an engine in which metallic contacts frequently occur.
Cr: Cr is an element in which the matrix is strengthened by dissolving therein and the wear resistance is thereby improved and in which hardenability of the matrix structure is improved. When the content of Cr dissolved in the Fexe2x80x94Cr-based alloy powder is under 2% by mass of the total mass of the Fexe2x80x94Cr-based alloy powder, the above effects are insufficient, and in contrast, when it exceeds 4% by mass, the compressibility is reduced by hardening of the powder, and therefore, the content of Cr is set to be 2 to 4% by mass.
Mo and V: Mo and V have an effect in which the matrix is strengthened by dissolving therein and the strength is thereby improved. When the content of Mo and V dissolved in the Fexe2x80x94Cr-based alloy powder is under 0.2% by mass to the total mass of the Fexe2x80x94Cr-based alloy powder, the effect is insufficient, and in contrast, when it exceeds 0.4% by mass, the compressibility is decreased by hardening of the powder. Therefore, the content of Mo and V is set to be 0.2 to 0.4% by mass, respectively.
Furthermore, it is preferable that the content of the Fexe2x80x94Cr-based alloy powder in the matrix forming mixed powder be 60% by mass or less. When it exceeds 60% by mass, the wear resistance is decreased by reduction of the area of Mo steel in the matrix, and in addition, the machinability is also reduced by increasing of a martensite phase.
In the case in which C is strengthened by dissolving in the matrix forming alloy powder, the compressibility is reduced by hardening of the alloy powder, and therefore, C is added in a form of graphite powder. C added in a form of graphite powder strengthens the matrix and improves the wear resistance. When the content of C is under 0.35% by mass, ferrite in which both the wear resistance and the strength are low remains in the matrix structure, and in contrast, when it exceeds 0.95% by mass, cementite precipitates at grain boundaries and the strength is reduced. Therefore, the content of added graphite is set to be 0.35 to 0.95% by mass of the total mass of a premixed powder.
The first hard phase formed by a first hard phase forming powder exhibits a form in which Mo silicide particles disperse in an alloy matrix of the first hard phase between Fe and at least one of Ni and Cr, and contributes to improvement in the wear resistance.
Mo in the first hard phase forming powder forms hard Mo silicide by binding mainly with Si, and contributes to improvement in the wear resistance by forming a core of the first hard phase. In addition, it also has an effect which firmly adheres the first hard phase to the matrix by dispersing in the matrix. When the content of Mo is under 10% by mass in the overall composition of the first hard phase forming powder, silicide is insufficiently precipitated, and in contrast, when it exceeds 50% by mass, the strength of the hard phase is reduced by the increase in the precipitated amount of the silicide, and therefore, parts thereof chip off during use and the chips act as a grinding powder and the wear amount increases. Therefore, the content of Mo is set to be 10 to 50% by mass.
Si in the first hard phase forming powder forms hard Mo silicide by binding with Mo as described above and contributes to improvement in the wear resistance by forming a core of the first hard phase. When the content of Si in the first hard phase forming powder is under 0.5% by mass in the overall composition of the powder, the silicide is insufficiently precipitated, and in contrast, when it exceeds 10% by mass, the compressibility is decreased by hardening of the powder and the adhesion to the matrix is deteriorated by firmly forming an oxide film on the surface of the powder. Therefore, the content of Si is set to be 0.5 to 10% by mass.
Cr and Ni in the first hard phase forming powder has an effect which strengthens the matrix of Mo silicide in the first hard phase and improves the hardness of the first hard phase, and an effect which prevents the Mo silicide from falling off, by adding at least one of the elements. In addition, it has an effect which improves the adhesion to the matrix structure by dispersing in the matrix structure. Therefore, it contributes to improvement of the wear resistance by these effects. When the content of Cr and Ni in the first hard phase forming powder is under 0.5% by mass in the overall composition of the powder, respectively, the above effects are insufficient. Furthermore, when the content of Cr exceeds 10% by mass, the compressibility is deteriorated by hardening of the powder and the adhesion to the matrix is reduced by firmly forming an oxide film on the surface of the powder. In addition, when the content of Ni exceeds 10% by mass, the compressibility is decreased by hardening of the powder and the wear resistance is deteriorated by austenitizing the matrix. Therefore, the content of Cr and Ni in the first hard phase forming powder is set to be 0.5 to 10% by mass, respectively.
When the content of the first hard phase forming powder having the above composition is under 5% by mass to the overall mass of the mixed powder, the amount of the first hard phase formed is insufficient, and it thereby does not contribute to improvement of the wear resistance. In the case of the second embodiment of a wear resistant sintered material of the present invention using only the first hard phase forming powder as a hard phase forming powder, when an amount of the first hard phase forming powder added exceeds 30% by mass to the total mass of the mixed powder, the wear resistant sintered material is hard; however, adverse effects occur such as decrease in the strength of materials, reduction of compressibility, etc., by increasing of a phase having a low toughness. Furthermore, in the case of the first or third embodiment of a wear resistant sintered member of the present invention using a second hard phase forming powder as described below as a hard phase forming powder, in addition to the first hard phase forming powder, when an addition amount of the first hard phase forming powder exceeds 25% by mass to the total mass of the mixed powder, the above adverse effects occur by a synergistic effect due to the two hard phase forming powders.
The second hard phase forming powder is used in order to disperse a second hard phase, in which a ferrite phase or a mixed phase of ferrite and austenite having a higher Cr concentration than that of a matrix structure thereof surrounds a core consisting of Cr carbide particles, in a matrix structure in the first or third embodiment of a wear resistant sintered member of the present invention.
Cr in the second hard phase forming powder forms Cr carbide with C in the second hard phase forming powder and contributes to improvement of the wear resistance by forming a core of the second hard phase. Furthermore, a part of Cr migrates to the matrix and acts to strengthen the matrix and the second hard phase, and it thereby contributes to improvement of the wear resistance of the overall sintered alloy. In addition, in an area having a high Cr concentration surrounding the second hard phase, a ferrite phase is formed and it thereby contributes to an effect which buffers impacts on a valve seating. When the content of Cr in the second hard phase forming powder is under 4% by mass in the overall composition of the powder, Cr carbide is insufficiently formed, and this does not contribute to the wear resistance. In contrast, when it exceeds 25% by mass, the amount of the carbide formed increases, and the wear of a facing member is increased and the compressibility is decreased by increasing of the hardness of the powder. In addition, the wear resistance is also reduced by increasing of the content of the mixed phase of ferrite and austenite. Therefore, the content of Cr in the second hard phase forming powder is set to be 4 to 25% by mass.
C in the second hard phase forming powder forms Cr carbide with the above Cr and contributes to improvement of the wear resistance by forming a core of the second hard phase. When the content of C is under 0.25% by mass in the overall composition of the powder, the carbide is insufficiently formed and does not contribute to improvement of the wear resistance, and in contrast, when it exceeds 2.4% by mass, the wear of a facing member is increased by increasing of the amount of the carbide formed and the compressibility is reduced by the increase in the hardness of the powder. Therefore, the content of C in the second hard phase forming powder is set to be 0.25 to 2.4% by mass.
In the above second hard phase forming powder, if at least one of, by mass, Mo: 0.3 to 3.0%, V: 0.2 to 2.2%, and W: 1.0 to 5.0% is contained, it is possible to further increase an effect of improvement of the wear resistance of the second hard phase.
Mo contributes to the improvement of the wear resistance by forming carbide with C in the second hard phase forming powder and by forming a core in the second hard phase which consists of the Mo carbide and the above Cr carbide. In addition, Mo which did not form the carbide has an effect in which high temperature hardness and high temperature strength of the second hard phase are improved by dissolving in the second hard phase. When the content of Mo in the second hard phase forming powder is under 0.3% by mass in the overall composition, the above effect is insufficient, and in contrast, when it exceeds 3% by mass, the wear of a facing member is enhanced by increasing a precipitation amount of the carbide.
V contributes to the improvement in the wear resistance by forming fine carbide with C in the second hard phase forming powder. Furthermore, the above carbide has an effect which prevents Cr carbide from coarsening, the wear of a facing member is suppressed and the wear resistance is thereby improved. When the content of V in the second hard phase forming powder is under 0.2% by mass in the overall composition, the above effect is insufficient, and in contrast, when it exceeds 2.2% by mass, the wear of a facing member is enhanced by increasing of a precipitation amount of carbide.
W contributes to the improvement in the wear resistance by forming fine carbide with C in the second hard phase forming powder. In addition, the above carbide has an effect which prevents the Cr carbide from coarsening, and the wear of a facing member is suppressed and the wear resistance is thereby improved. When the content of W in the second hard phase forming powder is under 1.0% by mass in the overall composition, the above effect is insufficient, and in contrast, when it exceeds 5.0% by mass, the wear of a facing member is enhanced by increasing of the precipitation amount of the carbide.
When the amount which is added of the second hard phase forming powder having the above composition is under 5% by mass to the total mass of the mixed powder, the amount of the hard phase which is formed is insufficient, and the second hard phase forming powder does not contribute to the wear resistance, and in contrast, even if it exceeds 30% by mass, not only is further improvement of the wear resistance not obtained, but also problems occur such as decreasing of the strength of materials, lowering of the compressibility, etc., by increasing of a ferrite phase which is soft and has a higher Cr concentration than that of the matrix structure. Therefore, the content is set to be 5 to 30% by mass in total mass of the mixed powder.
In the above metallographic structures of the first to third embodiments of a wear resistant sintered member of the present invention, it is preferable that a machinability improving component be dispersed in an amount of 0.3 to 2.0% by mass. As a machinability improving component, at least one of lead, molybdenum disulfide, manganese sulfide, boron nitride, calcium fluoride, and magnesium metasilicate mineral, can be employed. The machinability improving component serves as an initiating point of chip breaking in a cutting operation by dispersing in the matrix, and machinability of the sintered alloy can be improved.
Such machinability improving component is obtained by adding a machinability improving component powder consisting of at least one of lead powder, molybdenum disulfide powder, manganese sulfide powder, boron nitride powder, calcium fluoride powder, and magnesium metasilicate mineral powder in an amount of 0.3 to 2.0% by mass to the mixed powder. When the content of the machinability improving component, that is, the addition amount of the machinability improving component powder, is under 0.3% by mass, the effect is insufficient, and in contrast, when the content exceeds 2.0% by mass, the machinability improving component inhibits diffusion of powders during sintering, and thereby the strength of sintered alloy is lowered. Therefore, the content of the machinability improving component, (the addition amount of the machinability improving component powder) is set to be 0.3 to 2.0% by mass.
It is preferable that lead, lead alloy, copper, copper alloy, or acrylic resin be filled in pores of the above wear resistant sintered member. These are also machinability improving components. In particular, when a sintered alloy having pores is cut, it is cut intermittently; however, by having the pores filled with the above component, such a sintered alloy can be cut in a continuous manner, and this prevents shocks from being applied to the edge of the cutting tool. The lead and the lead alloy serve as a solid lubricant, the copper and the copper alloy serve to prevent heat from being accumulated and for reducing damage to the edge of the cutting tool by heating since thermal conductivity is high, and the acrylic resin serves as an initiating point of chip breaking in a cutting operation.
The machinability improving component can be filled by infiltrating or impregnating one of lead, lead alloy, copper, copper alloy, and acrylic resin, in pores of a wear resistant sintered member obtained by the above manufacturing process for a wear resistant sintered member.